I'm buying a house, and apparently I've got "dirty money"

I'm in the final stages of purchasing a home. My lender sent me this message after reviewing my bank statement:

Hi JstnPwll,

Thank you so much for sending everything. On the bank statement, there is a deposit $xxxx on 8-18 which needs to be sourced. Please let us know what that was so we can help you document it.

That was from a bitcoin sale I'd made last month to make sure I had plenty to cover all the inspections and closing costs. I found my Coinbase sale confirmation email and forwarded it, along with this note:

I believe that was when I sold some bitcoin at Coinbase. Here is the email confirmation from that sale, if that would help.

The follow-up reply:

Hi JstnPwll,

Funds from the sale of bitcoins cannot be used in a real estate transaction, due to anti-money laundering laws. You have plenty of other money available for closing, so this should not be an issue. Please sign the attached letter. We just need to acknowledge for the underwriter that this was not a loan & that you are not using this money in the deal since it is unregulated money.

After a bit of back and forth, I said:

Before I sign the form, I'd like to review the law in question just to make sure I fully understand the situation.

I haven't gotten a response yet. I wasn't aware of any real estate-specific law...do you guys know what the agent could be referring to?

Full disclosure: My money was not laundered.

Submitted September 29, 2015 at 06:17AM by JstnPwll http://bit.ly/1PKgrsA