Before giving a warning letter, have you given any verbal warnings? Have you explained the importance of collecting the deposit? Is your subordinate new to the job or experienced? Is this lapse occurring for the first time, or has it been repeated many times? Please consider all the above.
It is always beneficial in the long run to correct the reported issues by giving verbal instructions regularly and warnings rarely. If you still need a draft, read the following and modify it according to the facts.
Draft Warning Letter
"DEAR Mr...............
As per company procedure, you have to collect a deposit of ....... from every customer. On ........date, you failed to collect the deposit of ............. You did not inform our customer about........ Why did you not take the deposit? Since this is the first time, you are hereby given a warning.
OR
Since you have been repeatedly not following the procedures, you are hereby given a strict warning. You are strongly advised to always follow company procedures."
Regards,
V. Raghunathan