First, let me say that your trigger will cause you problems as it does not cater for multiple row updates, this is something you should look into changing as soon as possible.
Now, moving on to your immediate problem, this can be handled slightly differently.
My recommendation to you is to not attempt to do cross system updates within a trigger.
Better would be to write the updates to a local sql server database and process them outside of the transaction (the transaction that fired the trigger), this will remove the potential bottleneck every time you update your table.
Something like this would be your table to hold details of updates required to your MySQL table:
create table some_holding_table(inserted_time datetime default(getdate()) not null, ID int, Stock int);
GO
And your trigger would be something like this:
ALTER TRIGGER [dbo].[TriggerStock]
ON [dbo].[T_Contabilidad]
FOR UPDATE,insert
AS
BEGIN
INSERT some_holding_table(ID,Stock)
SELECT Idt_Conta,Stock FROM INSERTED
END
GO
Then you have a periodic process to read "some_holding_table" and perform the required updates to the remote system.
MM
select geometry::STGeomFromWKB(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