Why does Yang say that he cannot get married?

Scene 6 is set in "the 'private dining room' on the upper floor of a cheap restaurant in a poor section of town." Shen Te is there with her wedding guests: some of the family that has been taking advantage of her, the unemployed man, and Mrs. Shin. Yang Sun and Mrs. Yang, his mother, are removed from the crowd and have a secret conversation in which Yang reveals to his mother that Shen Te has said she can't sell the shop for him because of the loan from the old couple. His mother says, "Of course you can't marry her now," and he replies that since there is nothing in writing, he will try to reason with Shui Ta about it.