I've been responsible in the past for implementing AML systems for large multi-national banks and am familiar with AML laws not just in our country, but also with international anti-money laundering laws as well. I have to take training on it every year. Having said that, paying cash for property and writing a check against it almost immediately is also one of the most common ways to convert "dirty" money to clean. Not saying that's happening here, just saying it's one of the most common methods of money laundering.
In Manafort's case, this "investigation" may be a big nothing burger if Manafort has traceability to the money used to pay cash for the property. If he does, this "investigation" evaporates into thin air. Done, over, kaput.
In fact, most convertible HELOC agreements include check writing privileges.
I'm not fully fluent in HELOC agreements however don't those involve a loan for a property in which the borrower has equity in the property? That's not the case with Manafort. My understanding is no "loan" was made for the property, it was paid for in cash and that's a "red flag" for AML activities.
Every property purchased with cash has 100% equity for the owner...that was very much the case with Manafort.
A home equity line on such a property would be standard for any investor and those equity lines are typically convertible to a standard mortgage at the request of the customer.
Manafort could have easily bought a property for $1M cash, then tapped the equity line on it for $700K to finance his next cash purchase, then tapped the second property's equity line for $500K to finance his third purchase.
Completely legal and a common place practice for many investors.