Bitcoin wallet demanding source of funds

I took a 5-figure bitcoin payout from a sportsbook, had it sent to my bitcoin wallet, and then initiated a large sale. My bitcoin wallet contacted me and told me they want to talk to me, and know the source of the funds, before the sale will be completed and the funds will hit my account. In the meantime, I don't have access to those funds or the bitcoin. My question is, when I talk to them (assuming that's my only course of action), can I tell them the truth? Or will saying that the source of the funds was a sportsbook payout cause issues? Thanks!

Unless the gambling site is illegally operating, there shouldn't be a problem with it. If you can provide legitimate back-up to the payout like a receipt or confirmation, you're good.

They're probably on the lookout for much more illicit activity... drugs, guns, human trafficking. 

You say it gambling and you wont get paid. 

Tell them it’s marketing commissions.