Partial Payment posting in AR and clearing of invoice on fifo basis

Dear Gurus,

It is related to partial incoming payment in AR and clearing of invoices on FIFO basis, let me clarify my requiremnt through an Example

Assume there an ivoice of PKR 100,000 in customer Account XYZ

Company received partial PAyment of PKR 50,000 on specific date

Data entry clerk enters payment in F-28 in partial payment tab and assign it to invoice (please note that at present payment is not being assigned and both item invoice and payment remains open without any assignment with each other)

assume next payment which data entry clerk tries to enter with this refernce is PKR 60,000 while reamining amount with this invoice refernce is PKR 50,000 ,system should not allow to enter excess amount with partial paid invoice i.e. user has to be restricted to enter PKR 50,000 against this invoice

As data entry clerk enter partial payment and there is any other open invoice, system should not allow to enter payment till clearance of first open invoice

means specific requirement is

in case of partial payment with refernce to invoice, at the time of data entry system should chekc that no any other invoice is open in AR ledger

as partial payment document is entered system assign this document to Invoice

if data entry clerk tries to enter acess amount with refernce to invoice, system should not allow means Invoice amount = Payment amount

please advice procedure to meet this requirement

BR

sajida