PayPalから提供されているサンプルアプリを見たことがありますか。これがサンプルアプリのデリゲートメソッドです。これに従って、transactionIdを取得できます...
//paymentSuccessWithKey:andStatus: is a required method. in it, you should record that the payment
//was successful and perform any desired bookkeeping. you should not do any user interface updates.
//payKey is a string which uniquely identifies the transaction.
//paymentStatus is an enum value which can be STATUS_COMPLETED, STATUS_CREATED, or STATUS_OTHER
- (void)paymentSuccessWithKey:(NSString *)payKey andStatus:(PayPalPaymentStatus)paymentStatus
{
NSString* transactionId = [[NSString alloc] initWithString:[payKey substringFromIndex:3]];
status = PAYMENTSTATUS_SUCCESS;
[self messageAlert:[NSString stringWithFormat:@"%@",transactionId]
title:@"Transaction success" delegate:nil];
[transactionId release];
}