Attorney to Write Demand Letter - Minnesota

We need a very simple demand letter written on our business behalf to a client who is refusing to pay. We are an artist management company based in Minnesota, and our client is also in Minnesota. I've uploaded a copy of the agreement. He has paid so far a total of $2,500 of a $10,000 retainer. We agreed to a payment plan that reflected a payment of $5,000 on 05/15 that we graciously extended to 06/15. He has not paid anything, and is now telling us he won't pay. He recently got offered a very large recording deal worth 7 figures, and knows his contract requires him to pay 10%. He attempted to renegotiate the deal and cut us out of our commission, and we told him we were not willing to walk away from our commission. This is why he is refusing to pay the remainder of the retainer. Please put together a demand letter for the next payment due of $5,000. Must be paid within 3 business days of receipt.