The third party can bring ensuring performance of the contract

The court supported antimonopoly authority, having recognized such introduction of providing the contract lawful.

The creditor is obliged to accept the execution offered by the third party by the general rule. An exception is the situation when from the law, other legal acts, conditions or a being of the obligation follows that the debtor needs to fulfill the obligation personally. Such need doesn't follow from provisions of the Law N 44-FL.

Besides, the Law N 44-FL reserves the right to choose a way of providing the specified providing for the participant of purchase.

On a similar question there is also other point of view.