The scenario as such

1. You have a house on the market
2. A real estate company is interested in purchasing this house
3. They want to know what your payments are

Your questions
1. Why would they want to know your monthly payments

They may be interested in leasing the house back to you as you wait and search for a replacement property (if this is the case). They'ld probably then try to lease it to you at an amount close to your current payments. They may want to figure out your outstanding loan and make an offer to match just the loan if you are a distressed seller. I cannot think of any other reasons.

2. Can they find out how much you owe on the house?

They can search public tax records and find out what the last sale was. I do not think they can find your outstanding loans without running a credit check.