Nested for loop for non nested recordsets

My input xml is as under

<ns0:ProjectSync xmlns:ns0="http://XSLMapTesting.ProjectSync">
  <Funder>
    <OID>1</OID>
    <FFC>FFC1</FFC>
  </Funder>
  <Funder>
    <OID>2</OID>
    <FFC></FFC>
  </Funder>
  <Funder>
    <OID>3</OID>
    <FFC>FFC3</FFC>
  </Funder>
  <Funder>
    <OID>4</OID>
    <FFC></FFC>
  </Funder>
  <ActiveBudget>
    <BudgetLines>
      <Fund>
        <ID>1</ID>
      </Fund>
    </BudgetLines>
    <BudgetLines>
      <Fund>
        <ID>4</ID>
      </Fund>
    </BudgetLines>
  </ActiveBudget>
</ns0:ProjectSync>

My mapping xslt code is as under

<xsl:template match="/">
    <xsl:apply-templates select="/s0:ProjectSync" />
  </xsl:template>

  <!-- 
  ////////////////////////////////////////
  ProjectSync template
  ////////////////////////////////////////
  -->
  <xsl:template match="/s0:ProjectSync">
    <ns0:Projects>
      <Project>
        <Funders>
          <xsl:apply-templates select="Funder[*]" />
        </Funders>
        <Budgets>
          <xsl:apply-templates select="ActiveBudget/BudgetLines[*]" />
        </Budgets>
      </Project>
    </ns0:Projects>
  </xsl:template>

  <!-- 
  ////////////////////////////////////////
  Funders template
  ////////////////////////////////////////
  -->
  <xsl:template match="Funder">
    <Funder>
      <ID>
        <xsl:choose>
          <xsl:when test="FFC != ''">
            <xsl:value-of select="FFC/text()"/>
          </xsl:when>
          <xsl:otherwise>
            <xsl:value-of select="OID/text()" />
          </xsl:otherwise>
        </xsl:choose>
      </ID>
    </Funder>
  </xsl:template>
  
  <!-- 
  ////////////////////////////////////////
  BudgetLines template
  ////////////////////////////////////////
  -->
  <xsl:template match="ActiveBudget/BudgetLines">
    <BudgetLine>
      <FundID>
        <xsl:value-of select="Fund/ID/text()" />              
      </FundID>
    </BudgetLine>
  </xsl:template>
  
</xsl:stylesheet>

In the "BudgetLines template" above I want to apply a logic like for each budgetline it iterates through the Funders recordset and see if it finds a funder with same ID and also has a FFC populated, then FFC should be mapped. otherwise, Fund/ID should be mapped in the output xml. The same logic is also defined in the following code:

for-each select="BudgetLine"
  for-each select="Funder"
    choose
      when test="BudgetLine/Fund/ID = Funder/OID & Funder/FFC != ''"
        <xsl:value-of select="Funder/FFC/text()" />
        break;
      otherwise
        <xsl:value-of select="BudgetLine/Fund/ID/text()" />

The expected output for the above xml should be as under

<ns0:Projects xmlns:ns0="http://XSLMapTesting.Projects">
    <Project>
        <Funders>
            <Funder>
                <ID>FFC1</ID>
            </Funder>
            <Funder>
                <ID>2</ID>
            </Funder>
            <Funder>
                <ID>FFC3</ID>
            </Funder>
            <Funder>
                <ID>4</ID>
            </Funder>
        </Funders>
        <Budgets>
            <BudgetLine>
                <FundID>FFC1</FundID>
            </BudgetLine>
            <BudgetLine>
                <FundID>4</FundID>
            </BudgetLine>
        </Budgets>
    </Project>
</ns0:Projects>

how can I achieve this in xsl please? Thank you

If I understand your requirements correctly, I would suggest you do something like:

XSLT 1.0

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>

<xsl:key name="funder" match="Funder" use="OID" />

<!-- identity transform -->
<xsl:template match="@*|node()">
    <xsl:copy>
        <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
</xsl:template>

<xsl:template match="Funder">    
    <xsl:copy>
        <ID>
            <xsl:choose>
                <xsl:when test="FFC/text()">
                    <xsl:value-of select="FFC"/>
                </xsl:when>
                <xsl:otherwise>
                    <xsl:value-of select="OID"/>
                </xsl:otherwise>
            </xsl:choose>
        </ID>
    </xsl:copy>
</xsl:template> 

<xsl:template match="ActiveBudget">    
    <Budgets>
        <xsl:apply-templates/>
    </Budgets>
</xsl:template>   

<xsl:template match="BudgetLines">   
    <xsl:variable name="funder" select="key('funder', Fund/ID)" /> 
    <BudgetLine>
        <FundID>
            <xsl:choose>
                <xsl:when test="$funder/FFC/text()">
                    <xsl:value-of select="$funder/FFC"/>
                </xsl:when>
                <xsl:otherwise>
                    <xsl:value-of select="$funder/OID"/>
                </xsl:otherwise>
            </xsl:choose>
        </FundID>
    </BudgetLine>
</xsl:template>    

</xsl:stylesheet>

The code could be compacted somewhat if your processor supports XSLT 2.0 or higher.

(Added)

Actually, even in XSLT 1.0 you could reduce:

            <xsl:choose>
                <xsl:when test="FFC/text()">
                    <xsl:value-of select="FFC"/>
                </xsl:when>
                <xsl:otherwise>
                    <xsl:value-of select="OID"/>
                </xsl:otherwise>
            </xsl:choose>

to:

            <xsl:value-of select="(OID | FFC/text())[last()]"/>

and likewise for the other xsl:choose instruction.