Further to what I said above, this is an extract from the legal sector guidance which applies to solicitors supervised by the Law Society of Scotland:
"6.17.2.1 Establishing Source of Funds:
...
In circumstances where a client declares that they have been given funds for a transaction from a third party you (i.e. solicitor) may wish to record information relating to that original transaction too.
You may verify this by requesting bank statements and other relevant documentation relating to this transfer.
[Source of Funds] can often be difficult to determine without some understanding of the source of wealth of the individual. This can particularly be the case where the funds for a transaction have become mixed with other funds in an account. Here, to understand the SoF, you may need to have an awareness of the [Source of Wealth] of the individual, although your level of confidence in the source of wealth in such a case, should be considered on a risk-based approach."
Personally, I think the solicitor is missing the wood from the trees and creating a rod for her own back and her client's back (particularly at this late stage of the transaction) if it is clear that the OP can afford the deposit independently of the 2,500. She should ask what the deposit relates to but it is in her discretion, on a risk-based approach, to relax the requirement for evidence since OP is not relying on that deposit. The solicitor is not a detective trying to spot crime thousands of miles away in the US for an individual who is not even her client.
This is just my opinion, of course.
Those calling for a detailed investigation of the OP's uncle will be grateful that the ML regulations take a proportionate risk-based approach otherwise I assume they will equally be understanding if their own solicitor turns the interrogation spotlight on their finances.