3 sentences

1. 既然你已经应了下来，就不能反悔。

   jì​rán nǐ yǐ​jīng yīng le xià​lai, jiù bù néng fǎn​huǐ.

   Since you've already agreed to it, you can't go back on your word.

   Source: Dong Chinese Dictionary

2. 既然你已经答应了，就不能再反悔。

   jì​rán nǐ yǐ​jīng dā​ying le, jiù bù néng zài fǎn​huǐ.

   Since you have already promised, you cannot go back on your word.

   Source: Dong Chinese Dictionary

3. 为了防止对方反悔，他决定抢先一步签下合同。

   wèi​le fáng​zhǐ duì​fāng fǎn​huǐ, tā jué​dìng qiǎng​xiān yī bù qiān​xià hé​tong.

   To prevent the other party from backing out, he decided to strike first and sign the contract.

   Source: Dong Chinese Dictionary