What “Promise” Did Trump Make to Ukraine?

Cheriss May/NurPhoto via ZUMA Press

The Wall Street Journal reports today that President Trump asked the Ukrainian president “about eight times” in a single phone call to investigate Joe Biden’s son. This was obviously something Trump was pretty obsessed with. However, the Journal report also says this:
Mr. Trump didn’t mention a provision of foreign aid to Ukraine on the call, said this person, who didn’t believe Mr. Trump offered the Ukrainian president any quid-pro-quo for his cooperation on any investigation.
I’m pointing this out because yesterday’s reporting on the whistleblower complaint said the whistleblower was concerned about an alarming “promise” Trump made. This contradicts that reporting.
It’s possible that there are multiple phone calls involved here, or that some of the sources for the past week’s revelations don’t have the full story. Still, there’s a contradiction here.