I have one form where all client outstanding list are there.. FORM A

One another form is for Receipt Entry .. FORM B

Generally I fetch the Invoice no from Form A in Form B using (for each command). Against receipt we select the Invoice no and on submit the Invoice no mark paid in form A. 

Now, I stuck at one point if some one give part payment of any particular Invoice then I have to manually modify the entry in form A. 

Is there any way where I can fetch the Amount of each invoice in Text box where I can modify it and that will get update in form A. 

Thanks