Part payment of document with FPY1

Dear Experts,

I am processing outgoing payments through FPY1 for the Business partners. I have requirement where i need to make the partial re-payment of the advance received from BP through FPY1.

In case, if there is one document with an amount 1000/- i am able to process through FPY1 for the total amount.

but here my requirement is only process 400/- out of 1000/- through FPY1, please suggest how could i meet my requirement through FPY1.

Thanks for your help in advance

Aditya