The requirement for the ticket as expressed in federal regulations (at 8 CFR 217.2(c)) is
... a round trip ticket that will transport the traveler out of the United States to any other foreign port or place as long as the trip does not terminate in contiguous territory or an adjacent island; except that the round trip ticket may transport the traveler to contiguous territory or an adjacent island, if the traveler is a resident of the country of destination.
You are not a resident of Jamaica, so the exception does not apply to you. That means that your ticket must not terminate in Jamaica. But it doesn't; it terminates in Germany.
(You may be able to book this itinerary on a single ticket. At your favorite booking site, instead of "one way" or "round trip," choose "multi city" or similar.)
Earlier, at 8 CFR 217.2(a), the term round-trip ticket is defined thus (with some portions removed that are irrelevant to your case):
Round trip ticket means any return trip transportation ticket in the name of an arriving Visa Waiver Pilot Program applicant on a participating carrier valid for at least 1 year.... A period of validity of 1 year need not be reflected on the ticket itself, provided that the carrier agrees that it will honor the return portion of the ticket at any time, as provided in Form I-775, Visa Waiver Pilot Program Agreement.
So if your flight to Jamaica and/or to Germany is on a carrier that does not participate in the VWP, you might have trouble, though that seems unlikely in practice. The list of "signatory carriers" may be found at https://www.cbp.gov/document/report/signatory-visa-waiver-program-vwp-carriers. The list is extensive, but it seems that it must be taken with a grain of salt; for example, I cannot find Iberia on the list, but I am certain that they participate in the program.
