At the end of their 2-year mandatory sustainment period, they can ask the Regional Center to return their capital once they have filed their I-829 petition to remove the conditions from their green card.

You might want to review this link published by the USCIS:

https://www.uscis.gov/policymanual/HTML/PolicyManual-Volume6-PartG-Chapter5.html#footnote-4.

Scroll down and read Footnote 4 that says,(see 8 CFR 216.6(c)(1)(iii)) the sustainment period is the investorís two years of conditional permanent resident status. USCIS reviews the investorís evidence to ensure sustainment of the investment for two years from the date the investor obtained conditional permanent residence. An investor does not need to maintain his or her investment beyond the sustainment period.