Peace be upon you.
Grand Ayatollah Sayyid Sistani (may Allah protect him):
If the hiring is explicitly for direct action—either stated clearly or implied by the context—the hired person cannot hire someone else to perform the fast, nor can anyone else volunteer to do it for him. However, if the hiring is general, he may hire someone else (or someone else may volunteer to do it for him).
Grand Ayatollah Sayyid Khamenei (may Allah protect him):
In the case presented, if the fast is accepted with the condition of direct action or is implied to require it, volunteering for him does not suffice.
May you be successful in all good.