案例分析 Facts：
Last December Trevor decided to buy a second-hand car.He looked at a number of cars which his friend Joe, a car mechanic, checked for him before Trevor made his final choice.In order to buy the car Trevor borrowed £12,000 from his stepfather Damien.They agreed an annual rate of interest of 5% and that Trevor would repay the loan over three years by equal monthly installments of capital and interest, payment to be on the first day of each month.In January, Trevor lost his job and could not afford the full monthly repayments on the loan.Damien, mindful of the fact that it would be better for him to get some money rather than none, agreed to forego the interest.Trevor has continued to repay the capital.At the beginning of March, Trevor managed to get some well-paid employment.He did not tell Damien.When Damien eventually found out about the new job last week he immediately telephoned Trevor and said that, as from the end of June, Trevor would have to resume paying interest, including the arrears from 1st February.A few weeks ago Trevor telephoned Joe and told him that the car was running really well and that he had always meant to give him something for all of his help.Trevor said he would give Joe £100 when he next saw him.Joe said that he was not expecting any payment but that the money would certainly come in handy.Trevor does not think he should have to pay the increased monthly payments to Damien or the arrears of interest.He is also having second thoughts about paying £100 to Joe and wonders whether he is bound to do so.Advise Trevor 要求：
请结合我国相关法律规定，对案件进行分析，答题语言为英文。字数不少于 800 字，不能照抄法条，注意内在逻辑关系。
本案中，Trevor的朋友Joe帮助Trevor在购买汽车时把关的行为，由于Joe没有向Trevor收取任何费用，也没有签订任何相关合同，属于我国民法上的好意帮助行为，事后，由于Joe帮助把关购买的汽车状况良好，Trevor为了感谢Joe提出给予Joe100英镑的行为属于我国民法上的赠与行为，双方之间形成赠与合同法律关系。
赠与合同只需要赠与人有将自己的财产无偿赠与给受赠与人的意思表示，受赠与人表示接受即可成立。案例中Trevor通过电话向Joe做出了明确的意思表示，Joe也表示接受，因此他们之间的赠与合同法律关系已经成立，后来Trevor考虑到他每个月要支付更多的钱以及之前所欠的利息给Damien时，Trevor想要不给Joe支付这100英镑报酬的行为，是一种撤销赠与合同的行为，按照我国法律规定：赠与人在赠与财产的权利转移之前可以撤销赠与，但是具有救灾、扶贫等社会公益、道德义务性质的赠与合同或者经过公证的赠与合同不能撤销。显然，Trevor对Joe的赠与合同并不属于法律明确禁止撤销情形中的任何一种，Trevor可以在交付这100英镑之前撤销赠与合同。因此，Trevor没有义务必须给Joe100英镑。
案例中，由于Trevor在一月失业后无法向Damien足额偿还借款额度，Damien向Trevor表示同意放弃利息，是Damien放弃自己民事权利的行为。本质上是一种变更合同的行为，Damien变更合同的行为，按照我国法律规定，经过双方当事人的同意，并且没有违反相关的法律规定，合同可以进行变更，因此，免除利息的变更属于有效的合同变更行为。六月份当Damien得知三月初Trevor重新找到了一份高薪工作后，要求Trevor重新偿还自2月以来的利息不符合我国法律规定，因为Damien已经同意放弃2月份以后的利息。虽然3月Trevor重新找到了高薪工作，情况发生了变化，继续履行放弃利息的合同对Damien不公平，但是，这种变化并不是当事人在订立合同时无法预见的或者不可抗力在成的不属于商业风险的重大变化，而是一个可以预见的正常变化。因此Damien不能要求Trevor支付2月份以来的利息，Trevor也没有义务支付2月以来的利息。
