77 Pa. Stat. § 1507
On or after the seventh day after disability shall have begun or death shall have occurred, the employer and employe or his dependents may agree upon the compensation payable to the employe or his dependents under this act; but any agreement made prior to the seventh day after the disability shall have begun or the death shall have occurred, or permitting a commutation of payments contrary to the provisions of this act, or varying the amount to be paid or the period during which compensation shall be payable as provided in this act, shall be wholly null and void. It shall be unlawful for any employer to accept a receipt showing the payment of compensation when in fact no such payment has been made.
All agreements made in accordance with the provisions of this section shall be in writing, and signed by all parties in interest.
All agreements for compensation and all supplemental agreements for the modification, suspension, reinstatement, or termination thereof, and all receipts executed by any employe of whatever age, or by any dependent to whom compensation is payable under section three hundred and seven, and who has attained the age of sixteen years, shall be valid and binding unless modified or set aside as hereinafter provided.
77 P.S. § 1507