Unique situation with a possible Tenant

[Stephen Bruce]

Hello to all! Here is the scenario. A young couple (both 25) are relocating to my area and are interested in renting. The monthly rent is $2050/month. Their combined income is 90K. I did an in-person walkthrough, and the couple is very nice, responsible, clean and are extremely interested in renting. With their combined income, they barely meet the 3x criteria (when taking into consideration HOA and utilities) but they said the home they are selling will be giving them a large lump sum of cash. Here are the few areas where I don't know what to say:

1. They offered to pay the entire years rent upfront. Do I take that? Is that a bad idea? They said they'll have enough from the selling of their current home.

2. Their tentative move in date works great with our schedule but it is contingent on the sale of their home. In the lease do I write a clause that backs me up if they have to back out because their home didn't sell? How much do I charge them to get out of their lease? 

Overall, I'm very confident they will make great tenants! I hope to work with them, I just need advice on the above mentioned. Thanks!