Only the bold needs reading. Other text is for context if one needs it.
Nōn respuit condiciōnem Cæsar, iamque eum ad sānitātem revertī arbitrābātur, cum id quod anteā petentī [sibi] dēnegāsset ultrō pollicērētur; magnamque in spem veniēbat prō suīs tantīs populīque Romānī in eum beneficiīs, cognitīs suīs postulātīs fore utī pertināciā dēsisteret.
...that (thing) which, when it was being requested, he (Ariovistus) had previously denied [to him (Caesar)]...
It is meant to be a very literal translation. Am I correct in thinking that pententi is an abl. abs. as a temporal clause?