In testimony Friday, a Drug Enforcement Administration agent told a Delaware jury the president’s son, Hunter Biden, got swindled by his own crack dealer.

Mr. Biden is charged with lying about his drug use on a form while buying a gun in 2018. In federal court on Friday, the prosecution called witnesses to the stand to shine light on drug culture in America and crack cocaine specifically.

A supervisory special agent for the DEA, Joshua Romig, said that Mr. Biden paid “pretty significantly higher than the fair market value” for his crack cocaine.

Mr. Biden allegedly paid $600 for 10 grams of crack. Mr. Romig suggested that the dealer read Mr. Biden as someone willing to overpay for the drug, adding that dealers “100 percent” size-up what people are willing to pay and added that Mr. Biden was “clearly upset that the amount is much less than he paid for,” after the fact.

In a text message discussed at trial, Mr. Biden expressed dissatisfaction with the deal, sending a photo with a white substance on a scale saying “under any value known to any market in the world what I paid is 60 percent greater.”

Mr. Romig also discussed coded language Mr. Biden used in alleged drug deals, like calling drugs “baby powder,” which Mr. Romig said is a “rudimentary, pretty standard cocaine code.”

A federal prosecutor, Derek Hines, also asked Mr. Romig to describe the differences between crack cocaine and powder cocaine to the jury. He characterized crack as a “more easily smokable form of cocaine” that is cooked to remove any substances with which the cocaine may be cut.

In a cross-examination of Mr. Romig, Mr. Biden’s attorney, Abbe Lowell, wanted to focus on the discussion of the specifics of the case against Mr. Biden “and not generalities.”

During the cross-examination, Mr. Romig said that there was no indication, in his opinion, that Mr. Biden was distributing crack cocaine and that he never worked on Mr. Biden’s case as a DEA agent.

Earlier in the morning, a forensic chemist, Jason Brewer, testified on the scientific evidence against Mr. Biden. Mr. Brewer is a former FBI chemist who has testified as an expert witness in a myriad cases.

During his testimony, Mr. Brewer said that the white power found in a brown leather pouch owned by Mr. Biden tested positive for cocaine.  “I found a minimal amount of white powder,” Mr. Brewer said. “Cocaine was identified within the residual white powder that I sampled.”

Prosecutor Leo Wise also asked Mr. Brewer whether a test exists that could determine when a drug was placed in the pouch and who placed it there. Mr. Brewer said that no such test exists.