I recall a similar instance and the workaround was to make a dummy booking using the 50K certificate at a hotel where a standard room was 50K. Make sure it is refundable.

Once that 50K certificate is out of the picture, then it will let you use the smaller one for the reservation you do want.

Then cancel the 50K room you don't want.

Not sure if that still works but it did at the time the question was asked.