When it appears on the hearing of any scire facias, founded on a foreign attachment, that the debt due the principal is not yet payable, the court shall direct the execution to be stayed until the time the debt would have become payable; and when the debt was not payable at the time the demand was made on the execution, the court shall allow the garnishee his reasonable costs for defending against such scire facias, to be deducted out of the debt so due.