bought = purchased with money in the market. In Act_1:18, the word is not agorazo, as here, but ktaomai = acquired as a possession by purchase. Act_1:18 refers to quite another transaction. See App-161. There is no "discrepancy" except that which is created by inattention to the Greek words used.
with = out of. Greek. ek.App-104.
field. Greek. agros, not chorion = a small holding, as in Act_1:18.
to bury strangers in = for (Greek. eis.App-104.) a burying ground (Greek. taphe. Occurs only here) for foreigners.