drop table G

create table G ( 
    ref int, 
    glabel char(10), 
    inst char(10), 
    dev char(3) 
)

insert into G(ref, glabel, inst, dev) values(10, ' ', ' ', 'USD')

select * from G

drop table I

create table I ( 
    ref int, 
    gkey int 
)

insert into I(ref, gkey) values(123, 10)

select * from I

drop table M

create table M ( 
    ref int, 
    dev char(3), 
    gen int 
)

insert into M(ref, dev, gen) values(-1, 'USD', 1)

select * from M

merge into M 
using ( 
    select I.ref,  
           G.dev,  
           G.glabel 
    from I join G on I.gkey = G.ref 
) S 
on (M.dev <> ' ' 
    and M.dev = S.dev 
    --and case when M.gen > 0 then ' ' else '-1' end = S.glabel 
    ) 
when matched then update set M.ref = S.ref

commit

select * from M