Is an agreement on bids, whereby the bid is awarded to a person who pays specific sums to satisfy all bidders, and in the event that the bid is not awarded to him, the money paid is returned to him, is this money forbidden or permissible?

We understand from your question that it is impermissible to agree with all present bidders in an auction not to increase the price, so that the item is awarded to you at a low price in exchange for paying them sums of money, and it is impermissible to reclaim these sums if the item is not awarded to you.

What the scholars have permitted is for a person to ask some of those present, not all of them or most of them, to refrain from increasing their bids. If he agrees with some on this in exchange for a sum, he is not entitled to reclaim that sum if the item is not awarded to him.

Sheikh Al-Dardir says: "It is permissible for someone present at an auction who wishes to buy an item to ask some of those present, not all of them, to refrain from increasing their bids so that the asker can buy it cheaply, even if for compensation; such as, 'Refrain from bidding higher, and you will have a dirham.' The compensation becomes obligatory whether he buys it or not... It is not permissible to ask all, or most, or a single person who is in the position of a group, such as the market master."

Based on this, the aforementioned sums in the question are not lawful for the recipient, because they were paid in an illegitimate manner.