Cash Receipt application to multiple accounts / unapplied cash open

Let us explain this with a scenario:

The business received an amount of 10000$ from different customers. They created a receipt of 10000$ with customer column at header level as blank. In the Line level, we apply the receipt to different customers. Once we saved the receipt, the customer column at header level gets filled up automatically with the first customer invoice applied.

So the user is asking that at header level, the customers should be showcased only for the invoices applied against in the receipt. That means in the receipt if customer A invoice is applied then at header level the customer column should show as A. If customer B invoice is applied then at header level the customer column should show as B only.