Beneficiary is one who unintentionally gains a benefit from a contract between other parties

The answer is "incidental beneficiary".

An incidental beneficiary refers to somebody who indirectly acquires an advantage as the aftereffect of the fundamental reason for the trust. An incidental beneficiary is a recipient who isn't a planned recipient. For instance, a grandchild may profit by his/her parent accepting a blessing which could be utilized by the whole family, or which he/she may acquire from the parent.