A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence, and coupling strength. High coherence can be attained by building circuits from fixed-frequency qubits; however, leading techniques cannot couple qubits that are far detuned. Here, we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183-ns two-qubit iswap gate between qubits separated in frequency by 854 MHz, with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface-code circuits and for analog quantum simulation.