There are not significant difference between two sentences. Don't let it confuse you. Usually, "之" is widely used in ancient written Chinese and it is also used nowadays. You are confused because someone told you that you cann't say ... Maybe the only rule for this is : when you use two characters, choose 的， when you meet one character, use 之, but in most case, you can also use 的, not the reverse。 for example, 受朋友之“恩”=受朋友的“恩惠”。 受人之托 is more like an idiom. we rarely use such structure in other cases. so just remember this.