Please check(self-edited)

Sanjay, I can't help but to put my business hat on for this one.  A shrewd business person isn't likely to be in such dire financial straits that he has to borrow from personal friends or family.  He should be able to borrow from a bank.  If a bank won't loan him money, it is because the risk of default is too great.  If he could easily tackle the financial crisis, the risk would be low or he could negotiate a more favorable repayment schedule with his vendors. 

I have assumed that the loan is for running a business, since you mentioned him being a businesman.  If it is a personal crisis, such as unexpected illness, that could be a different matter.  But still, a shrewd businessman usually saves for a rainy day as well. 

You may decide to lend him the money, but be aware that loaning money to friends and family is discouraged.  A significant percentages of such loans are never repaid and relationships are often permanently damaged.  I personally learned this lesson when I was younger.  I loaned money to a few people at various times.  One person repaid the loan in regular installments and that went well.  One person paid when I forced the issue and the relationship ended.  The others never paid and I lost both the money and those relationships.  Two of those were "at your convenience" arrangements.  Apparently, it never became convenient. 

Having said all that, I'll assume that you will still loan the money and need help in writing the letter.  My thoughts on your three edits:

I agree that hear works better than know. 

For the reasons explained above, I just really don't like the references to him being shrewd, that he can easily tackle the issue, or that it won't be a big deal.  Is there another reason you are loaning him the money?  Perhaps you have faith that he will find a way to climb out of his situation. 

Either "at your convenience" or "when you are financially comfortable" are interchangeable to me.  Still, I discourage an open-ended loan.

That's more motherly business advice than grammar advice, but I hope it helps.