Wounded and dishonourably discharged from the Prussian Army, Major von Tellheim finds himself waiting at a Berlin hotel with his servant for the outcome of his trial, threatened by financial troubles and serious bribery allegations. His penniless condition is because repayment of a large sum advanced to the government during the recent war is being held up and his honor in making the loan questioned. During Tellheim's absence from the inn, the landlord has caused Tellheim's effects to be removed, ostensibly because his rooms were needed for a lady and her maid. In reality, the landlord doubts Tellheim's ability to pay, since he is already somewhat in arrears.
In the removal of the Major's possessions, the landlord comes upon a sealed envelope marked as containing five hundred thalers. This discovery makes him anxious to placate Tellheim. What he does not know is that the money has been left with the Major by Paul Werner, his former sergeant. Werner knowing Tellheim's predicament is in hope that he will use the money as his own. Tellheim is too honorable to borrow when he has no assurance of repaying. Instead, he bids his servant, Just, take his last possession of value, an expensive ring, and pawn it to satisfy the landlord's bill and his own back wages.
Just pledges the ring with the landlord but refuses to accept either wages or dismissal on the plea that he is in Tellheim's debt and will have to work it out. The garrulous landlord shows the ring to his newly arrived guests, revealing considerable concerning the owner's circumstances. The lady, Minna von Barnhelm, recognizes the ring as one of the betrothal rings which she and Tellheim had exchanged, and is overjoyed that her search for her missing lover is ended.
When Tellheim appears, however, he refuses to accept her hand or to continue the engagement on account of his precarious circumstances. When no argument can move him, Minna with the help of her maid, Franziska, pretends that she, too, is penniless and in dire straits. Under these circumstances Tellheim immediately claims the privilege of marrying and protecting her.
At this point a delayed letter from the King is delivered. It announces the restoration of Tellheim's fortune and the vindication of his honor. To punish him for making her suffer, Minna now pretends that she cannot marry Tellheim because of the inequality of their circumstances. In answer to his pleas, she uses his own recent arguments to confound him. Only when Tellheim is reduced to the verge of despair and the belated arrival of Minna's uncle and guardian threatens to give the whole thing away does Minna relent and reveal the truth. In a final scene of celebration, matters are settled to the satisfaction of everyone including Franziska and Paul Werner who have discovered a lively interest in each other.

Question: Why does Tellheim change his mind about not marrying Minna?
Answer: he is tricked into believing she is poor like him
Alternative answer: She pretends to be poor and in terrible circumstances.