Mu Lianmiao had been sleeping in Musheng's bed, and to avoid suspicion, she shared the bed with Ma Changying. Otherwise, even if they were not sharing the same bed, Ma Changying would still be somewhat worried, because she had been observing how Musheng and Mu Lianmiao's relationship was becoming increasingly intimate lately. However, she did not know that Musheng indeed had already been in the most intimate relationship with Mu Lianmiao.
For Ma Nanji's birthday, Musheng and Mu Lianmiao gave him the same present—money.
Simple, crude, and the most direct.
Choosing gifts is troublesome and might not even please Ma Nanji, but giving him money was the most straightforward approach. Just by seeing that moron grinning from ear to ear, one could tell how successful it was.