第一句：An acceptance once given cannot be revoked unless the offer consents.
除非要约人同意，承诺一经做出即不得撤回。

A: If I want to modify the contract, what should I do?
如果我想修改合同，我该怎么做？
B: An acceptance once given cannot be revoked unless the offer consents.
除非要约人同意，承诺一经做出即不得撤回。
A: Yes.
是的。

第二句：I mean a contract may be modified if the parties reach a consensus through consultation.
我的意思是如果当事人协商一致即可以变更合同。

A: I mean a contract may be modified if the parties reach a consensus through consultation.
我的意思是如果当事人协商一致即可以变更合同。
B: So I should communicate with my partner.
那么我要和我的合作伙伴沟通一下。
A: Yes, of course.
是的，当然了。
B: Thank you.
谢谢。

其他表达法：

Traditionally, firm referred to a partnership, as opposed to a company.
按惯例，firm是指合伙企业，与公司相对。

A transaction between two parties ought not to operate to the disadvantage of a third.
双方合约不约束第三人。

合伙协议应当载明的事项：入伙与退伙；合伙企业的解散与清算；违约责任；经营期限；合伙人争议的解决方式。