I used to work in transfer pricing for 5 years so I know a bit about international tax (I am not an international tax expert by any means). Tax authorities don't generally care about transactions unless they're adversely impacted. US tax laws are usually written to say that intercompany transactions should happen at an arm's length price (i.e. fair market value).
If you sell your EU company to your US company for $1, the US tax authorities will be happy. The US taxes worldwide income, so the income of your EU company will become taxable in the US forevermore, all for the low price of $1.
Companies typically shift intellectual property from high tax jurisdictions (e.g. the US) to low tax jurisdictions (e.g. Ireland). Shifting IP to the US at below market prices is the opposite of what companies like Google does to minimize taxes.