@AnnetteM
In your Gallery OnSelect, set the following formula:
Set(glbCurrentRecord, ThisItem); Reset(yourFormNameHere)
On your Add button/Icon, set the following formula:
Set(glbCurrentRecord, Defaults(yourDataSourceNameHere)); Reset(yourFormNameHere)
In the DefaultMode property of your form, set the following:
If(glbCurrentRecord.PaidStatus = "Paid", FormMode.View, FormMode.Edit)
Substitute names accordingly above.
I hope this is helpful for you.