Have you considered doing it as a “reservation” Movement instead?
Reservation would work as Karen suggested. You could also use the trial adoption functionality - turn it on under Settings->Options->Movements. It allows a "Trial" tickbox when recording adoptions and you can choose to treat trial adoptions as on shelter. You can also use the Move->Trial Adoption Book screen to see them. When you want to make a trial permanent, just untick the box on the movement.