I'm afrad our new dog will try 2 hurt my dog?

our family just adopted a grayhound from a rescue place that only helps grayhounds and our new dog is really nice and we all love him! when we went to go look at the grayhounds they said thay got along really good with other dogs witch was important becuz we have another small dog daisy. so our new dog sammie gets along really good with daisy and cuddels with her and everything but when we put them in the backyard to play sammie tries to bite daisy and when daisy runs away it gets worse. when sammie bites her she cries realy loud but then growls and bites him bak and sammie stops. but then 5 seaconds later when daisy tries to get away he runs after her and bites her again. but he only does this outside. when they are on leash there is no problem. pleeese help! we dont get why he keeps doig this! thay get along every other time but just when thay are in the bakyard. im afriad he will hurt daisy becuz he seems really mean when chasing her and she tries to hide under our porch and bites him bak.