Hi Sarva,
You can use Cumulative Sum functoid with second parameter as "1".
I have created one sample xsd from the sample u provided. Used below instance for testing:
<ns0:ZPAYMENT_EXTRACT xmlns:ns0="http://sample_dynamicXSLT.input_idocSchema">
<IDOC>
<BEGIN>BEGIN_0</BEGIN>
<EDI_DC40>
<field1>field1_0</field1>
<field2>field2_0</field2>
</EDI_DC40>
<Z1PAYHDR>
<SEGMENT>Girish</SEGMENT>
<ACCOUNT>111</ACCOUNT>
<XMIT_DATE>XMIT_DATE_0</XMIT_DATE>
<Z1PAYDTL>
<PAYMENT_DATE>PAYMENT_DATE_0</PAYMENT_DATE>
<AMOUNT>100</AMOUNT>
<CREDIT_CARD>ABC</CREDIT_CARD>
</Z1PAYDTL>
<Z1PAYDTL>
<PAYMENT_DATE>PAYMENT_DATE_0</PAYMENT_DATE>
<AMOUNT>200</AMOUNT>
<CREDIT_CARD>PQR</CREDIT_CARD>
</Z1PAYDTL>
<Z1PAYDTL>
<PAYMENT_DATE>PAYMENT_DATE_0</PAYMENT_DATE>
<AMOUNT>300</AMOUNT>
<CREDIT_CARD>XYZ</CREDIT_CARD>
</Z1PAYDTL>
</Z1PAYHDR>
<Z1PAYHDR>
<SEGMENT>Naveen</SEGMENT>
<ACCOUNT>222</ACCOUNT>
<XMIT_DATE>XMIT_DATE_0</XMIT_DATE>
<Z1PAYDTL>
<PAYMENT_DATE>PAYMENT_DATE_0</PAYMENT_DATE>
<AMOUNT>40</AMOUNT>
<CREDIT_CARD>ABC</CREDIT_CARD>
</Z1PAYDTL>
<Z1PAYDTL>
<PAYMENT_DATE>PAYMENT_DATE_0</PAYMENT_DATE>
<AMOUNT>80</AMOUNT>
<CREDIT_CARD>PQR</CREDIT_CARD>
</Z1PAYDTL>
</Z1PAYHDR>
</IDOC>
</ns0:ZPAYMENT_EXTRACT>
Got the output as:
<ns0:ZPAYMENT_OUT xmlns:ns0="http://sample_dynamicXSLT.output_idocSchema"><CUSTOMER>
<SEGMENT>Girish</SEGMENT>
<ACCOUNT>111</ACCOUNT>
<TOTALAMOUNT>600</TOTALAMOUNT>
</CUSTOMER>
<CUSTOMER>
<SEGMENT>Naveen</SEGMENT>
<ACCOUNT>222</ACCOUNT>
<TOTALAMOUNT>120</TOTALAMOUNT>
</CUSTOMER>
</ns0:ZPAYMENT_OUT>