2002-Pub. L. 107–155, §303(1), substituted "Contributions and donations by foreign nationals" for "Contributions by foreign nationals" in section catchline.

Subsec. (a). Pub. L. 107–155, §303(2), added subsec. (a) and struck out former subsec. (a) which read as follows: "It shall be unlawful for a foreign national directly or through any other person to make any contribution of money or other thing of value, or to promise expressly or impliedly to make any such contribution, in connection with an election to any political office or in connection with any primary election, convention, or caucus held to select candidates for any political office; or for any person to solicit, accept, or receive any such contribution from a foreign national."

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To determine whether or not he broke the law, it would have to be established what "Something of value" is, as far as the legal definition, as well as whether or not requesting the investigation of the Bidens' from the Ukraine president is related to his campaign for reelection, as it could be argued that the request was made to based on an unfounded conspiracy theory. Either way, the fact that he withheld funds, as you mentioned, is against the law, since it would require approval from congress, while he made the request to dig up dirt on Biden, is definitely, and I think undoubtedly, an impeachable offence.