When you make the change to asking price to absorb a credit, which is rare but sometimes needed, it is a simple "wash".
Let's say it's $5,000 of rebate to be absorbed and the purchase price is $300,000 with the seller having signed a contract pre-listing to offer the Agent for the Buyer 3% of the sale price. In that case the Agent for the Buyer signs something with the Seller changing that commission to "3% minus $5,000" and the Buyer and Seller sign something changing the price to $295,000. These have to be done simultaneously. One agreement is signed by the agents (brokerages) both and the seller, and not the buyer. The other is signed by the buyer and seller and not the agents. It takes both documents to complete the exchange of $5,000 of commission for a $5,000 reduction in the agreed purchase price.
That means both brokerages and the buyer and the seller all have to sign...and consequently it is often a "last resort" and rarely the method used to pick up a small gap in agreed credit vs allowed credit. If the credit is $5,000 but the disallowed credit/overage is only twenty five bucks, as example, it's easier for the agent to give the buyer a $25 gift certificate to Home Depot as a "closing gift", than to involve all of the parties in effecting the absorption of a small amount.
A large check after closing is most often allowed when the buyer's downpayment is in excess of the amount "required". Let's say the interest rate and the loan approval is the same for 35% down as it would be for 30% down and the buyer is putting 35% down. In that case the lender would likely allow the credit or cash after closing. Not because they allow the agent to pay part of the 35% down money, but because they can easily call that same dollar amount downpayment 30% down instead of 35% down, with no adverse consequence. They are not approving the cash credit at or after closing, as much as they are approving the loan on the same terms with 30% down vs 35% down. They net the credit from the downpayment, with 30% down being more than sufficient.
I appreciate that you feel my responses are "profound", and thank you, but it's really just basic math, knowing the WHY of the rules that apply...plus a dash of everyday common sense. 
It's every day real estate. Not only knowing "the rules" but how they apply differently in each case at hand. It's a dance...one day it's a cha-cha...next client it's a rhumba. Many different ways to end up at the same good result.
There is no "ethical" issue...it sounds more like there is some confusion. Part of the problem is that the Buyer Agent Fee is actually, by contract prior to the home being listed, promised to the Listing Brokerage..who agrees to pay X of the total to the Buyer's Brokerage, if there is one. That means if there is no Buyer's Brokerage or the Buyer's Broker charges less than the amount offered, the difference belongs to the Listing Brokerage by contract with the seller put in place before the home is listed. Consequently an Agent for the Buyer is somewhat limited as to what they can promise their client, the buyer, by the contracts already in play before the buyer enters the room. That is why in most cases, the Buyer Agent credit is handled as a matter of disbursement instruction, vs any other method that requires seller approval, when and if possible.
If it needs to be a Purchase Price reduction, the agent usually knows that at the time they write the offer, and must add a clause saying that the credit can be used toward closing costs OR as a price reduction, in whole or in part. In other words, you need the seller to be on board with that option possibly being needed, at time of offer...and before the loan documents get to escrow...and not at the last minute. If the purchase price change changes the loan amount...you need to get that all nailed down before the lender draws the loan documents, or with enough time for the lender to send new loan documents.
What surprises me most about local escrow companies is they often do not alert anyone when there is an excess credit. They simply negate it down to the amount needed to close, without telling anyone there is "a problem". The buyer getting less and the seller getting more...is typically not viewed as "a problem" by escrow companies. So you have to pay very close attention, or you get stuck at closing signing away your "difference" to the seller.
