Okay, I just checked Richter, and what he says on page 20 is that „in addition to the Zernickow estate, which he administrated exemplarly (...), Fredersdorf later bought other estates, and even became, as we know through the earlier mentioned document of Count Schlitz, the owner of a „colony“ in East India.“ Ostindien is British India, „Kolonie“ is written in quote marks by Richter, which doesn‘t indicate irony - Richter doesn‘t do irony - but presumably an old fashioned use of the term. For example, the French huguenot settlement in Berlin was called „die Kolonie“. Conversely, maybe he does mean a plantation - at a guess, tea or pepper, givne India‘s most common exports. Either way, I could be wrong, but I don‘t think British India did slaves in the second half of the 18th century. (Though the slave trade certainly was still going strong with British traders to other colonies.) No mention of South America or Mexico, or any other overseas possession. The next sentence is about beer.

ETA: Oh, I see you already found the sentence. The „earlier mentioned document“ refers to the one in which Fredersdorf‘s widow doesn‘t sign herself „von“, which is how Richter concludes Fredersdorf wasn‘t ennobled. Also, belatedly it’s occured to me that before the demise of the East India Trading Company it‘s not British India yet, but in terms of 18th century use of the term „Ostindien“ in the fictional texts I‘ve seen it in, it was employed for the continent and not the islands. Again: „I could be wrong“ caveat, big time.