Why are you accepting payment in cash? Get a cheque from the client instead, and then deposit that in your bank account (if you don't have a bank account, now is a good time to get one). Once the cheque has been cashed, send a letter to the client stating that the cheque has been processed and you can begin work.
Something like this may be useful (bear in mind it's a Sunday and I'm not in my "business mode" so I may be off by a bit).
(123 Your Street Address)
(Your City, ST 01234)
(Name of Client)
(456 Their Street Address)
(Their City, ST 98765)
Re: (Name of Project)
Acct # (XXXX-XXXX-XXXX-XXXX) (This will be the account number for the job; if there is no account number, then don't use this.)
Amount: ($X,XXX.xx) (This will be the amount of the payment that was made.)
Dear (Client Name):
This letter is being sent to you in response to a payment for (describe the services to be provided) dated (Date), which was received on (Date). Your payment has been received and processed. (I/We) look forward to working with you to complete your project as soon as possible. If you have any questions please do not hesitate to contact (me/us) via (list the methods you wish to be reached by for normal business inquiries and so forth) between the hours of (list your business hours). In the event of an emergency, (I/we) can be reached via (list the methods you wish to be reached by in case of emergency).
Please retain this letter for your records. In the event you lose this letter, a copy will be sent to the above address upon request.
(Your Signature - actually write this out, don't print or type)
(Your Name - Type this out in your word processor program of choice)
[Edit: One other thing, make sure you have a signed CONTRACT before accepting ANY money. If this is what you were looking for, just ask me and I'll direct you to some online resources to help you out.]