高利贷（usury）的法律比较复杂

现在的问题是，即使合同合法，你们所在的州是否有限制高利贷的法律，以至于合同无效。这个要看你在哪个州。

比如在加州， http://oag.ca.gov/consumers/general/usury
1. The California Constitution allows parties to contract for interest on a loan primarily for personal, family or household purposes at a rate not exceeding 10% per yea

2. In transactions for the purchase of goods or services which are not for personal, family or household purposes, there are normally no limits to finance charges except those set by the parties. 这个应该适用于你的情况，你是基本上是生意贷款（炒股），应该没有高利贷限制。所以你不必因为内疚而不收利息。

当然，如果你不在加州，请自己google本地的法律。比如纽约的法律又不一样 http://kdklaw.com/whats_new_02_private-loans.htm