issue changing partially billed sales order currency

Hello Experts,

I have this issues to resolve base on my client requirement:

1. My client uses Milestone Billing Plan and do Billing 60% and 40%. After Billing 60%, my client wants to be able to Bill the remaining 40% occasionally in a different currency. If the document currency was in NGN for instance, they want to be able to occasionally change the Document Currency to USD for the remaining 40%. Their reason being that they want to be able to print invoice in the changed currency if need be. This is generating error message on the Sales Order "Currency not permitted" because the Sales Order has been partially billed.

I was thinking of a workaround to better change the Currency at the Invoice level since the Currency Change is because of the Invoice Printout, do you think this will solve the problem? I have not been able to test this workaround because the Currency Field on the Billing Document is Grayed-Out, how can I un-grey the field?

Please I need your expert guide on the best way to resolve this issue and satisfy my client requirement. Please make your guide detailed if possible.

Great Regards,

Patrick